Godtha˚bsfjord region north of Paamiut shows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north 550 km consists of a mosaic of ductile fault-bounded packages that attained
their present relative positions in the late Archaean. © 2001 Elsevier Science B.V. All rights reserved.
Keywords
:
Archaean; Zircon, U – Pb geochronology; Gneiss complexes; Terrane tectonics
1. Introduction
Understanding how high-grade gneiss terrains were constructed is hampered by polyphase, het-
erogeneous high strain and polyphase metamor- phism which normally obliterates most primary
textures and structures. This makes the identifica- tion of cogenetic rock suites very difficult. Fur-
thermore, during these processes, whole-rock and mineral isotopic systems are commonly disturbed.
Geochronological studies of rockes so affected is fraught with difficulty, and may lead to geologi-
cally meaningless results e.g. Hamilton et al. 1979 cf. Kinny and Friend, 1997, Burton et al.
1994 cf. Whitehouse et al. 1996. Ion-mi- croprobe U – Pb single zircon dating allows indi-
vidual samples to be more reliably dated so that when dealing with complicated field relationships,
analytical results can be directly compared rather than attempting to correlate using lithological
similarities. Application of this technique to the Godtha˚bsfjord region of southern West Green-
land Fig. 1 e.g. McGregor et al., 1991; Friend et al., 1996, confirmed the concept that different
terranes sensu Coney et al., 1980, had been amalgamated in the late Archaean, an hypothesis
first suggested on a structural and metamorphic basis Friend et al., 1987, 1988; Nutman et al.,
1989. Because the Godtha˚bsfjord region com- prises only a part of the West Greenland Ar-
chaean,
it is
important for
the overall
understanding of the evolution of the North At- lantic craton to know whether terrane assembly
can be recognised over a wider area. The Paamiut region Fig. 1 was mapped in the
late 1960s the region being interpreted as a single block, with a more or less uniform history
throughout e.g. Berthelsen and Henriksen, 1975; Higgins, 1990; Kalsbeek et al., 1990. A new field
study of this region McGregor and Friend, 1997, had two main aims. First, to test whether
the Paamiut region was indeed a continuous block of similar history, as indicated on existing maps
e.g. Allaart,
1975, 1982
or, like
the Godtha˚bsfjord region to the north, was con-
structed from individual blocks with different tec- tono-thermal histories e.g. Friend et al., 1988.
Second, to search for outcrops of early Archaean gneisses that, from whole-rock Pb – Pb isotopic
work on Proterozoic mafic dykes which traverse the region, were postulated to occur at depth
Kalsbeek
and Taylor,
1985. In
the Godtha˚bsfjord region such rocks, the Itsaq Gneiss
Complex, have distinct field characteristics e.g. Nutman et al., 1996. In a global context, it would
be important to identify another area where early Archaean rocks occur. However, no definitive
field or isotopic evidence was found to suggest that early Archaean rocks occur in the Paamiut
region. This paper presents the zircon geochronol- ogy carried out to establish the ages of some of
the crustal components and metamorphic events identified by McGregor and Friend 1997.
2. Pre-1992 investigations of the Paamiut region
2
.
1
. Field work Geological mapping of the Archaean gneiss
complex in the Paamiut region was completed Allaart,
1975 when
Archaean quartzo-
feldspathic gneisses were thought to be largely of sedimentary or volcanic origin e.g. Ramberg,
1948; Sutton and Watson, 1951; Berthelsen, 1960; Kalsbeek, 1970; Andrews, 1973. This view is no
longer accepted and it is understood that the gneisses originated as plutonic igneous rocks
Kalsbeek et al., 1990. However, the geology was still presented as one continuous block e.g. Hig-
Fig. 1. Sketch map of the geology between Bjørnesund in the southern part of the Tasiusarssuaq terrane and the boundary of the Ketilidian mobile belt showing the newly recognised tectonic boundaries. Location of Fig. 2 is indicated. Insets: a Location of the
Archaean craton in Greenland; box indicates location of b. b Western side of the Archaean craton showing division into different terranes. Box indicates position of Fig. 3.
gins, 1990; Kalsbeek et al., 1990 containing undi- vided, typical Archaean tonalite, trondhjemite,
granodiorite TTG
suite gneisses
from the
Ketilidian boundary north to Frederiksha˚b Is- blink Fig. 1. Kalsbeek et al., 1990 considered
that the rocks north of Paamiut were similar to those north of Frederiksha˚b Isblink, and conse-
quently concentrated on describing the rocks be- tween Paamiut and Ivittuut. The gneiss protoliths
were thought to have formed between 3000 – 2850 Ma and to have been metamorphosed under am-
phibolite facies conditions in the late Archaean. No evidence for granulite facies metamorphism in
the region was reported e.g. Higgins, 1990; Kals- beek et al., 1990 and Refs. therein. Within the
TTG
gneisses, amphibolite
and gabbro-
anorthosite units
occur as
semi-continuous marker units or as trains of enclaves. Locally,
units of sillimanite-bearing rocks, interpreted to represent pelitic to semi-pelitic compositions have
been recognised e.g. Rivalenti and Rossi, 1972. The Tartoq Group Higgins and Bondesen, 1966;
Higgins, 1968 is a separate, thick sequence of greenschist and low amphibolite facies rocks
derived from pillow lavas, other volcanic and associated sedimentary rocks. On the basis of its
lower metamorphic grade and structural evidence the group was thought to be younger than the
main gneisses Berthelsen and Henriksen, 1975.
2
.
2
. Isotopic data Few isotopic data exist for the Paamiut region.
In a whole-rock Pb – Pb isochron study, Taylor and Kalsbeek 1986 examined three units. Am-
phibolite facies, grey, biotite-bearing gneisses from the south-west Vesterland Fig. 1 yielded a
date of 2784 9 53 Ma, interpreted as a minimum protolith age. At Kuummiut Fig. 1 grey, banded
gneisses gave 2985 9 115 Ma, and a younger, intrusive trondhjemite component was dated at
2769 9 110 Ma. The same trondhjemite was dated using SHRIMP U – Pb single zircons, with
dates of 2922 9 4 Ma 2s for prismatic grains with later zircon overgrowth at 2827 9 11 Ma.
These were interpreted to date the protolith and a high-grade metamorphic event respectively Nut-
man and Kalsbeek, 1994. Igneous zircons from a tonalitic sheet cross-cutting the Tartoq Group
were dated at 2944 9 7 Ma Nutman and Kals- beek, 1994. This demonstrates that at least parts
of the Tartoq Group is older than previously supposed c.f. Berthelsen and Henriksen, 1975
and appears to be different from the supracrustal units found further north as suggested by Higgins,
1990.
3. New field evidence
